1. Field of the Invention
The present invention relates to an article of manufacture. In particular, electronic circuits provide radio frequency signal processing means.
2. Discussion of the Related Art
Cable television services are delivered to the home typically via a single cable that connects to a power divider such as a cable television signal splitter for sharing the CATV service with multiple rooms in the home. The need to serve multiple locations leads to use of the splitter and with it an amplifier to boost the CATV signal before it is divided. Notably, externally powered amplifiers cease to operate when there is a power outage unless there is a source of backup power. Packages or “boxes” that contain the splitter and the amplifier are sometimes referred to as CATV “drop amps.”
With the advent of voice over internet protocol (“VOIP”) telephony, drop amp modifications have included the addition of an input splitter that divides out a VOIP port before the signal is split for CATV use in the home. This segregation of the VOIP port enables VOIP service to continue irrespective of whether there is a power outage affecting the drop amp amplifier.
In order for the VOIP signals to be delivered to the home correctly, the input splitter needs to maintain a high return loss (low reflection coefficient) in the situation when the amplifier is not powered. Most CATV amplifiers are designed to have a high input return loss over 18 dB when DC power is on which results in low reflections of the signal. This 18 dB return loss relates to an impedance of about 75 ohms+/−3 ohms when the amp is powered but becomes a poor return loss of 8-10 dB when power is removed. The problem exists in the design of commonly used Wilkinson hybrid splitters where the input return loss can only be high if the output ports also have a proper impedance termination. If the splitter output port return loss feeding the amplifier goes low, so does the input to the splitter and thus unacceptable system operation. An acceptable splitter input return loss needs to be in the 18 dB range for proper VOIP operation and thus, due to the hybrid splitter design, one output may have a termination between 60-90 ohms to insure the splitter input return loss is high enough. To resolve this problem of a power outage reducing the input splitter return loss, a circuit needs to be located, for example between the splitter output port and amplifier input port, to maintain a constant impedance to the splitter port despite amplifier impedance changes. The problem becomes difficult to solve as the needed impedance adjustment is required as power is removed thus making it difficult to effect a change.
Prior art teaches impedance matching requires either an active detection and adjustment circuit, a mechanical relay in the energized state to release and insert a new termination with loss or power, or a radio frequency (“RF”) MOSFET switch that can have a low RF insertion loss with no power and a high insertion loss with power.
Prior art U.S. Pat. No. 5,629,653 teaches an active circuit to detect and adjust a variable capacitor to adjust the load. Prior art U.S. Pat. No. 6,531,936 teaches the use of a varactor diode as the adjustable tuning element fed by an active detector. Prior art U.S. Pat. No. 4,769,618 shows the use of a MOSFET switch to switch in loads as needed to effect a constant termination in varying conditions.
Prior art U.S. Pat. No. 7,974,586 teaches a mechanical relay that when de-energized switches from an amplifier connection to a 75 ohm resistor to ground. As skilled artisans will recognize, reliance on mechanical devices leads to reliability problems. Another known switching method noted in the '586 patent is use of an RF MOSFET (as also used in U.S. Pat. No. 4,769,618) to selectively switch to a 75 ohm load when system power is removed. Substituting a mechanical switch for an electronic switch improves reliability but it also increases cost.
FIG. 1 shows a prior art drop amp without a segregated VOIP port 100. Here, a drop amp case 101 has multiple ports including a CATV input port 102, CATV output ports 112, and an external power supply connection 113. Inside the case, a filter block 114 provides for frequency multiplexing, a feature that enables a feed cable 130 from the cable system 132 and an output cable 140 to a home appliance 142 such as a television to transport signals bidirectionally.
Frequency multiplexing is provided by a filter block input diplexer 116 and a filter block output diplexer 126. The input diplexer includes a high frequency filter 160 transferring signals to a high frequency filter 162 of the output diplexer. The output diplexer includes a low frequency filter 152 transferring signals to a low frequency filter of the input diplexer 150.
An input signal line 104 interconnects the input port 102 with the input diplexer while output signal lines 110 extend between respective output ports 112 and an output splitter 108. A filter output line 106 interconnects the output diplexer 126 and the filter output splitter.
Between the input and output diplexers 116, 126, high and low frequency signals 163, 153 are transported on high and low frequency channels. In CATV systems, the high frequency channel is typically the frequency band 54-102 MHz transporting CATV program content while the low frequency channel is typically the frequency band 5-42 MHz transporting home originated content such as user instructions directed to the CATV system.
Within the filter block, the input diplexer 116 demultiplexes the incoming CATV program signal 163 from the signals at the drop amp input 102 and multiplexes the outgoing home originated signal 153 with the signals at the input. As shown, the high frequency signal 163 is transported to an amplifier 120 by amplifier input line 122 and from the amplifier to the output diplexer 126 by an amplifier output signal line 123. And, as shown, an amplifier is typically used in the high frequency channel and not in the low frequency channel. Explanations for this may include increased attenuation of high frequency signals and local origination/stronger signal of the low frequency signal.
Also within the filter block, the output diplexer 126 multiplexes the amplified incoming signal 163 with the signals at the filter output line 106 and demultiplexes the home originated signal 153 from the filter output line. As shown, the low frequency signal 153 is transported to the input diplexer 116 via a low frequency line 118.
FIG. 2 shows a prior art drop amp including a segregated port such as a VOIP port 200. Here, an input power divider such as a directional coupler 204 divides the input signal to provide the segregated or VOIP port 210.
The input splitter 204 interconnects with the drop amp input port 102 via an input signal line 202. Splitter divided signal lines 206, 208 interconnect with the filter block input diplexer 116 and with a segregated port 210. The segregated port provides bidirectional signal transport such as voice signals originating in the home 213 (transmitted voice) and voice signals originating outside the home 212 (received voice).
The drop amp of FIG. 2 provides VOIP service that does not rely on power provided to the drop amp by the external power supply 103. This feature recognizes the need to maintain certain services, such as telephone service, even when there is a power outage. However, this drop amp configuration creates an out of specification impedance problem for cable television distribution systems. The problem appears when the drop amp amplifier loses power and changes impedance as a consequence. For example, an operating amplifier with a nominal 75 ohm input impedance undergoes an impedance change such as an impedance increase and return loss “dip” when power is lost.
Such a return loss dip increases the strength of reflected signals and, among other things, impairs VOIP service. CATV operators therefore require that loads connected to the CATV distribution system, such as drop amplifiers, maintain a nominal 75 ohm input impedance.
Various solutions to the return loss “dip” problem associated with power loss to drop amps have been designed. Typical of these prior art solutions is the circuitry found in U.S. Pat. No. 7,974,586 to Romerein et al. (the “'586 patent”) included herein by references in its entirety and for all purposes.
FIG. 4 of the '586 patent, shown here as FIG. 3A, depicts a relay switched solution to the return loss dip problem. Here, a relay selectively interconnects the non-VOIP directional coupler output with either of the filter block input or with ground via a resistor 206, for example a nominal 75 ohm resistor. Operating the relay to interconnect the coupler output to ground when there is a power outage (as shown, contacts 209 open and 210 closed) and operating the relay to interconnect the coupler output to the filter block input when power is available (not shown, contacts 209 closed and 210 open) provides a fix for the return loss dip problem.
FIG. 8 of the '586 patent shown here as FIG. 3B depicts an electronic switched solution to the return loss dip problem. Here, a solid-state switching circuit 230 that is equivalent to an electromechanical single-pole-double-throw relay is connected between diplex filter 204 and the input of amplifier 216. The manner of operation follows. Whenever power is applied to amplifier 216, it is also applied to input terminal 218, of the solid-state switching circuit 230, causing the pin diode 254 to pass RF signals from bypass capacitor 244 through bypass capacitor 258 to the input of amplifier 216. Also, when power is applied to amplifier 216 in the solid-state switching circuit 230, MOSFET switch 108 is turned off, electrically disconnecting resistor 252 from input terminal 231. When power is lost or dropped out from amplifier 216, it is also removed from power terminal 218 of the solid-state switch 230, causing the pin diode 254 to become backbiased, preventing signal flow from diplex filter 204 through the solid-state switch 230 to the amplifier 216. Also, as previously discussed, when power is lost, MOSFET switch 108 operates to lower the resistance of its main current path for effectively connecting resistor 252 between input terminal 231 and ground. (Col. 7, ll 44-60)
When power is lost, this switched solution interconnects the input diplexer high frequency connection with a resistor to ground. When power is available, the switch interconnects the input diplexer high frequency connection with the amplifier input.